Ultrasonic flow measuring devices are used often in process and automation technology. They enable contactless determining, especially in a pipeline, of the volume- and/or mass-flow of a medium.
Known ultrasonic flow measuring devices work either on the basis of the Doppler principle or on the basis of the travel-time difference principle. In the case of the travel-time principle, the different travel times of ultrasonic measuring signals, in the stream direction and counter to the stream direction of the medium, are evaluated. For this, the ultrasonic measuring signals of the ultrasonic transducers are alternately emitted and received, in the stream direction and counter to the stream direction of the medium. On the basis of the travel-time difference of the ultrasonic measurement signals, the flow velocity, and therewith, at known pipe diameter, the volume flow rate, or, with known density of the medium, the mass flow rate.
In the case of the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium. Ultrasonic measuring signals reflected in the medium are evaluated. On the basis of a frequency shift arising between the in-coupled and reflected ultrasonic measuring signals, also here, the flow velocity of the medium, or, as the case may be, the volume- and/or mass-flow can be determined. The use of flow measuring devices working according to the Doppler principle is, however, only possible, when air bubbles or impurities are present in the medium, on which the ultrasonic measuring signals can be reflected. Therefore, the use of such ultrasonic flow-measuring devices is rather limited, in comparison to ultrasonic flow measuring devices working according to the travel-time difference principle.
Regarding the types of flow measuring devices, a distinction is drawn between ultrasonic flow measuring devices which are introduced into the pipe, and clamp-on flow measuring devices, in the case of which the ultrasonic transducers are pressed externally onto the pipeline by means of a clamping mechanism. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, and in the U.S. Pat. Nos. 4,484,478 and 4,598,593.
In the case of both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated into, and/or received from, the pipeline (in which the flowing medium is located) at a predetermined angle. In order to be able to radiate the ultrasonic measuring signals at a certain angle into the tube and, thus, into the medium, in the case of clamp-on, flow-measuring devices, the in- and out-coupling of the ultrasonic measuring signals into the pipeline occurs through a mediating element, e.g. a coupling wedge. In order to achieve an optimum impedance matching, it is, moreover, known to manufacture the coupling wedges from a suitably refracting material, e.g. of plastic. The main component of an ultrasonic transducer is usually at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals.
Furthermore, there is increasingly a requirement in process automation, that two-conductor devices be used. Two-conductor devices have, as compared to measuring devices in which the communication between measuring device and remote control location occurs, likewise as does the energy supply of the measuring device, over separated lines, the advantage that at least one line, or conductor-pair, as the case may be, is saved. By the saving of lines, costs and installation times are considerably lessened. Moreover, two-conductor devices are, because of their low energy consumption, best suited for use in explosion-endangered areas, which underlines their attractiveness compared with three, or four, wire devices.
However, in the case of two-conductor devices, there is the problem of covering the energy requirement of the measuring device, despite considerably lessened, available energy. As already indicated, in the case of ultrasonic flow measuring devices, the ultrasonic measuring signals are sent through the medium being measured and, moreover, in the case of clamp-on flow measuring devices, radiated through the pipe wall into the medium being measured, and then out of the medium being measured. Due to unfavorable impedance ratios, a major weakening of the ultrasonic measuring signals is experienced, so that the required amplification, depending on application, lies in the order of magnitude of 20-120 dB. The frequency of the ultrasonic measuring signals lies between about 100 kHz and 10 MHz. Electronic components, which work in this frequency range, require, as is known, relatively large currents, a fact which stands in the way of use of two-conductor technology in the case of ultrasonic flow measuring devices.